For more than 70 years Pluto was classed as a planet,until 2006,when the international astronomical union said it was a dwarf planet. They said that because they thought it was smaller than all of the other planets. It still orbits the sun but it is just smaller.

A man called Mike Brown thought that he had found a planet further than Pluto called Eris. Pluto did not meet the third criteria, so that is why it is classified as a dwarf planet.